Direct or indirect fired drying kilns are used to dry cut lumber. In a direct fired kiln, a fuel such as propane or natural gas is combusted at a burner to directly heat air which is circulated through a plenum or distribution duct which extends along the length of the kiln drying chamber. In an indirect fired kiln, a heat exchanger transfers heat generated by a heat source such as a burner to the air to be circulated through the kiln. The indirect fired kiln avoids circulating combustion products with the heated air. Vents are provided in the plenum through which the circulated heated air escapes to heat the interior of the kiln. These vents divide the drying chamber of the kiln into various zones. Presently, the vent openings through which the circulated air escapes are manually adjusted once at start-up in an attempt to ensure an even heat distribution along the length of the kiln drying chamber. Thereafter, there is no way of adjusting the distribution of heat in the kiln. Additionally, the lumber loaded into the kiln to be dried may be of varying moisture content. The wetter lumber should have hotter air directed to it, in order that it should dry to a moisture content equal to the rest of the lumber. With drier batches of lumber, the reverse is required.
The present invention provides an improved direct or indirect fired kiln in which the distribution of heat along the length of the kiln drying chamber can be automatically controlled to ensure that equal drying of lumber in the kiln is achieved.